Sounds almost like the lockbar and blade tang are binding a little bit. I'm assuming the lockup is very tight, with no play?
Just an idea, I haven't tried this myself, but have you tried some dry graphite powder in the mechanism? Seems like some people have had success using simple pencil lead (graphite) on the lockbar faces of framelocks, for similar reasons (sticking or binding, making the lock difficult to disengage). If trying this, it might be a good idea to clean up the knife thoroughly first, to remove any oils that might be in there. Don't know if the combined oil & graphite might gum up, or otherwise make a mess in the lock.
An afterthought:
The Case Cheetah is a mid-lock (lock is depressed in the central portion of the spine of the handle, as opposed to being further back), and sometimes those can feel pretty stiff anyway. Relatively short lever for the lock bar, which can make the spring underneath it feel pretty stiff at times. Do you have any other mid-lock folders to compare with?